Method of producing corundum layer on metal parts

ABSTRACT

A method of forming a corundum surface layer on a metal part has the steps of providing an electrolytic bath, immersing a metal part in the electrolytic bath as an anode, providing a cathode, and passing a wave electric current with a gradual increase of voltage between the cathode and the anode to 1000-1800 V.

BACKGROUND OF THE INVENTION

The present invention relates to an electrochemical process of application of coatings, and can be used in machine construction, instrument making, and medicine.

Methods of forming protective or decorative ceramic coatings on aluminum and their alloys are known. They are disclosed for example in U.S. Pat. Nos. 3,956,080; 4,082,626; 4,659,440; 5,275,713 and use an anodic process with application of direct current and voltage up to 600 v for obtaining ceramic coatings with high corrosion resistance and adhesion. They however have the disadvantage that the ceramic coating is formed slowly and has low hardness.

In U.S. Pat. No. 5,147,515 in order to apply coatings, electrophoresis is utilized with a electrolyte formed by colloid aqueous solutions of silicates or water-insoluble salts with high concentration, where ceramic particles are in suspended state. Direct current is used, which has a wave shape and preferably a rectangular shape, with a gradual increase of voltage from 50 to 200 v. Such coating can have a thickness up to 700 micron, high hardness, wear resistance and resistance to corrosion. The disadvantage of such coatings is a non-uniform thickness of the coating, high roughness of the surface, and low cyclical resistance to thermal shock, and also low adhesion.

U.S. Pat. No. 5,616,229 discloses the use of a wave electrical current as a current source, with a wave shape in form of a tooth, with voltage approximately 700 volt which is provided by a capacitor block with a total capacity 375 microfarad. In order to accelerate the process of forming the coating, salts of metals are added to the electrolyte. However, the addition of metal salts leads to an increase of the layer on the surface of the part and to a change of at least its size and increase of surface roughness. With the use of the capacitor block with a total capacity of 375 microfarad, it is possible to obtain coatings on small parts with the surface up to 0.1 dm².

U.S. Pat. No. 5,720,866 discloses the use of direct current in first and second phases, and the use of alternating current with superposition of direct current or with alternating square anodic pulses and square cathodic pulses with reduced amplitude in third phase, while in fourth phase the magnitude and frequency of cathodic pulses is reduced and the magnitude of anodic pulses is increased.

The existing methods can be further improved

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method of forming a corundum surface layer on metal parts, which is a further improvement of the existing methods.

More particularly, it is an object of the present invention to provide a corundum layer on a surface of parts composed of metals Al, Ti, Ta, Mg, steel and their alloys, with the use of an electrochemical method in a mode of anodic-cathodic, galvano-plasma treatment in alkali electrolytes, for obtaining heat resistant, wear resistant, erosion-resistant, and corrosion-resistant surfaces of parts.

It is also an object of the present invention to reduce a time of formation of a corundum layer, to reduce time of application of a coating, to increase resistance of parts to a thermal shock, and to provide erosion resistance.

In accordance with the present invention the above mentioned objectives are achieved by the use of an wave alternating current which has a gradual increase of voltage between a cathode and an anode to 1000-1800 volt, preferably to 1800 volt.

The inventive method can be performed with a changing time of anodic and cathodic pulses of voltage, executed in three modes.

First mode is a cathodic mode, in which time and amplitude of a cathodic pulse of voltage is increased and amplitude and time of anodic pulse of voltage is reduced. This mode is used for removal of metallic admixtures, fusible compounds and oxides from the surface layer of a part. Second mode is an anodic-cathodic mode, in which time of action and amplitude of anodic pulse of voltage is increased, and time of action and amplitude of a cathodic pulse of voltage is reduced. This mode is provided for formation of a corundum layer without changes in dimensions of the part. Third mode is an anodic-pulse mode, in which time and amplitude of anodic pulse of voltage is increased with a subsequent pause. This mode is used for accelerated formation of a corundum layer of a great thickness (up to 1 mm) with low porosity and high adhesion.

The treatment can be performed in an alkali electrolyte with an optimal concentration, with maintaining a ratio between alkali, liquid glass, catalyst and filler 1:2:0,2:2. The part which is immersed into the electrolyte can be fixed on an aluminum anode which can be arranged on an ultrasound vibrator. A renewed and cold electrolyte can be supplied into a zone of arc discharges with a speed of movement along a surface of the part from 100 to 1000 mm/sec.

The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a cathodic mode with an asymmetric pulse of voltage of the method in accordance with the present invention;

FIG. 2 is a view showing an anodic-cathodic mode with an asymmetric pulse of voltage and increased time of an anodic pulse, of the inventive method;

FIG. 3 is a view showing an anodic mode with an increased time of an anodic pulse of voltage and a pause, of the method in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to provide a corundum surface layer on a part in accordance with the present invention, a cathode composed of stainless steel or titanium alloy is introduced into an electrolytic bath, or a housing of the bath can be composed of these materials. A metal part is then introduced as an anode into the electrolytic bath, preferably it is arranged on a suspension of for example aluminum alloy with an outside insulation from electrolyte, and mounted in an ultrasound vibrator, to subject the part to ultrasound vibrations for obtaining a surface with low roughness.

The electrolyte can be composed of a solution in a distilled water of NaOH, KOH, NaAlO₂, KAlO₂, Na ₂SiO₃, K₂SiO₃, Na ₆P₆O₁₂, K₆P₆O₁₂, KMNO₄, Al₂O₃, fine powders of oxides, carbides and nitrides in required combination and concentration, depending on required properties of a corundum layer and a metal of the metal part. A ratio between alkali, liquid glass, catalyst and filler is selected to be 1:1:0.2:2.

A wave electric current is then turned on, which can be an asymmetric current, and voltage between the cathode and the anode is increased substantially to 1000-1,800 volt. This provides an exceptionally intense and efficient formation of the corundum layer on the metal part.

The application of the wave electric current is performed in three modes. First mode is a cathodic mode which is performed with a ratio of amplitude of anodic current to cathodic current less than 0.5 and frequency 50-60 Hz with pulses of voltage of equal time. During this mode removal of metal admixtures, fusible compounds and oxides from the surface of a metal part is performed. Second mode is an anodic-cathodic mode which is performed with a ratio of amplitude of anodic current to cathodic current from 1.2 to 5 with frequency 50-200 Hz and a ratio of time of anodic pulse to cathodic pulse of 1.2 to 2.5. During this mode a formation of a corundum layer without changes in a size of a metal part takes place, basically by converting a surface layer into the corundum layer. Third mode is anodic pulse mode with a pulse 0.2 of time of anodic pulse, with frequency 50-400 Hz. It provides an accelerated formation of a corundum layer of great thickness, low porosity, and high adhesion. In each mode the voltage is increased to 1000-1800 volt, preferably to 1800 volt.

The metal part is gradually immersed into the electrolyte with are voltage 380-400 volt, in order to increase the productivity. Then the metal part is immersed into the electrolyte completely to a depth for example of 10 mm from the surface of the electrolyte, in order to prevent corrosion of the surface of the metal part if it extends outwardly beyond the surface of the electrolyte.

The electrolyte is subjected to constant purification (filtration), from particles of oxides, non-metallic admixtures, impurities, etc. Composition and temperature of electrolyte is constantly controlled, and the electrolyte is mixed mechanically. Air or oxygen is also introduced into the electrolytic bath for enrichment of electrolyte with air.

In accordance with an important feature of the present invention, the cleaned, renewed and cooled electrolyte is supplied into arc discharges zone along a surface of the metal part with the speed 100-1000 mm/sec. This significantly intensifies the formation of the corundum layer. In accordance with another important feature of the present invention the electrolyte is cooled to the temperature 10-250, in order to prevent dissolving of oxides. Also, gaseous products of decomposition of electrolyte are removed.

In order to form a corundum surface layer over a surface of the metal parts, the following steps can be performed. A cathode formed of stainless steel with content of Fe less 30% or titanium alloy is introduced into an electrolytic bath, or a bath housing can be formed as the cathode. A metal part is introduced as an anode into the electrolytic bath on a suspension of aluminum or its alloy, mounted in an ultrasound vibrator.

In order to form a corundum layer on cast aluminum alloys, the electrolyte is formed as a solution in distilled water of 1-2 g/l NaOH, 2-4 g/l Na₂ SiO₃, 2-4 g/l Na₆P₆O₁₂, 2-10 g/l TiO₂. For forming a corundum layer on titanium metal parts or metal parts of titanium alloys, the electrolyte includes a solution in distilled water of 2-4 g/l NaOH, 4-8 g/l Na₂SiO₃, 4-8 g/l Na AlO₂, 2-10 g/l TiO₂. For forming a corundum layer on steel parts the electrolyte is a solution in distilled water of 5-50 g/l KAlO₂, 5-200 g/l of fine powder of carbide, nitride or oxide, 10-70 g/l Na₆ P₆O₁₂. Wave electric current is then applied with a gradually increasing voltage between the cathode and anode to 1000-1800 volt in anodic-cathodic and anodic-pulse modes.

Also, a fine powder can be used including 50 g/l Na₃ PO₄, 3-200 g/l V₂O₅, 3-200 g/l MnO₂, 3-200 g/l Cu₂O, 2-25 g/l TiO₂, 3-200 g/l Ni₂O₃, 5-150 g/l Na₂ CO₃, 3-200 g/l CuO.

In order to provide formation of corundum coatings with a great thickness up to 1 mm on a metal parts of steel, Al, Mg, Ti, Ta and their alloys the electrolyte is utilized, which is composed of a solution in distilled water of 1-2 g/l KOH, 5-20 g/l KAlO₂, 2-4 g/l K₂SiO₃, 5-200 g/l Al₂O₃, 5-10 g/l K₆ P₆O₁₂. The wave electric current is turned on with a gradual increase of voltage between cathode and anode to 1000-1800 volt in anodic-pulse mode with a pause approximately 0.2 of time of the anodic pulse of voltage, with frequency 50-400 Hz.

In order to obtain corundum-silicate coatings with a great thickness on metal parts composed of steel, Al, Ti, Ta, Mg and their alloys a wave electric current is passed with a gradual increase of voltage between the cathode and anode to 1000-1800 Volt with a current density over 100 A/dm² in the anodic-cathodic mode, with a ratio of anodic current to cathodic current from 1.2 to 5, with frequency 50-200 Hg, and a ratio of time of anodic pulse to cathodic pulse from 1.2 to 2.5. The electrolyte is used which is composed of a solution in a distilled water of 2-4 g/l NaOH, 100-150 g/l Na₂SiO₃, 5-10 g/l Na₆P₆O₁₂, and NaAlO₂.

In the cathodic mode a wave alternating cathodic electric current is formed by adding of alternating current of a first phase of a high voltage transformer converted into one half-period direct current, and a second phase of the alternating current of the high voltage transformer, while a wave alternating anodic electric current is formed from a third phase of the high voltage transformer of alternating current. This is shown in FIG. 1

In the anodic-cathodic mode a wave anodic electric current is formed by adding alternating current of a first phase of the high voltage transformer converted into one or two half-period direct current, and a second phase of alternating current of the high voltage transformer, while a wave cathodic electric current is formed from a third phase of the high voltage transformer of alternating current. A duration of the anodic pulse exceeds a duration of cathodic pulse 1.2-2.5 times. This is shown in FIG. 2.

In anodic pulse mode an anodic pulse current is formed by adding of alternating current of a first phase of the high voltage transformer converted into one half-period direct current, and alternating current of the second phase of the high voltage transformer converted into a one half-period alternating current, and alternating current of a third phase of the high voltage transformer converted into one semi period direct current connected with the cathode. This is shown in FIG. 3.

In order to speed up a start of the process of galvanic-plasmic formation of a corundum layer and to increase speed of formation of the corundum layer, in the electrolyte a mixture of NaAlO₂ up to 50 g/l and fine powder Na₆P₆O₁₂ up to 10 g/l are introduced.

In order to obtain a corundum layer with a low roughness the electrolyte can be composed of the solution in water of alkali, liquid glass and catalyst with the ratio 2:1:0.2, the anodic-cathodic mode is used with an increase of a frequency of pulses at the end of the process to 100-200 Hz, and the metal part is arranged on a suspension and fixed in an ultrasound vibrator to be subjected to ultrasound vibrations.

In order to increase a speed of formation of a corundum layer and to reduce required power, the cathode can be formed as a cup in which the anode is introduced and then a distance between the wall of the cup-shaped cathode and the anode representing the metal part is maintained to be substantially 20-40 mm.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in method of anodic-cathodic galvano-plasma treatment for forming corundum surface layer on metal parts, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 

1. A method of forming a corundum surface layer on a metal part, comprising the steps of providing an aqueous electrolytic bath; immersing a metal part in the electrolytic bath as an anode; providing a cathode; and passing a wave electric current with a gradual increase of voltage between the cathode and the anode to 1000-1800 V.
 2. A method as defined in claim 1, wherein said passing includes passing the wave alternating electric current with current density of substantially 100-150 amp/dm².
 3. A method as defined in claim 1; and further comprising using a cathodic mode for removing from a surface layer of the metal part admixtures, fusible compounds and oxides.
 4. A method as defined in claim 3, wherein said providing the cathodic mode includes maintaining a ratio of amplitude of anodic current to cathodic current less than 0.5 with frequency 50-60 Hz with an equal duration of voltage pulses.
 5. A method as defined in claim 1; and further comprising using an anodic-cathodic mode for forming the corundum layer without changes of sizes of the metal part.
 6. A method as defined in claim 5, wherein said anodic-cathodic mode includes a ratio of amplitude of anodic current to cathodic current 1.2-5 with frequency 50-200 Hz and a ratio of time of anodic pulse to time of cathodic pulse 1.2-5.
 7. A method as defined in claim 1; and further comprising performing an anodic pulse mode for forming a corundum layer with a great thickness, low porosity, and high adhesion.
 8. A method as defined in claim 7, wherein said performing includes performing the anodic pulse mode with a pause 0.2 of time of anodic pulse and frequency 50-400 Hz.
 9. A method as defined in claim 1; and further comprising performing an anodic-cathodic mode, and a subsequent anodic pulse mode.
 10. A method as defined in claim 1; and further comprising subjecting the metal part to ultrasound vibrations.
 11. A method as defined in claim 1; and further comprising supplying an electrolyte to a zone of spark discharge along a surface of the metal part with a speed 100-1000 mm/sec.
 12. A method as defined in claim 3, wherein in the cathodic mode a wave cathodic electric current is formed by adding alternating current of a first phase of a high voltage transformer converted into a one half-period direct current, and a second phase of alternating current of the high voltage transformer, and forming a wave anodic electrical current from a third phase of the high voltage transformer of alternating current.
 13. A method as defined in claim 5, wherein in the anodic-cathodic mode a wave anodic electric current; and further comprising in the anodic cathodic mode forming a wave anodic electric current by adding of an alternating current of a first phase of a high voltage transformer converted into one or two half-period direct current a second phase of alternating current from the high voltage transformer; and forming a wave cathodic electric current from a third phase of the high voltage transformer of alternating current, with a duration of an anodic pulse exceeding a duration of a cathodic pulse 1.2-2.5 times.
 14. A method as defined in claim 7; and further comprising in the anodic pulse mode forming an anodic pulse current by adding of an alternating current of a first phase of a high voltage transformer converted into a one half-period direct current, and an alternating current of a second phase of the high voltage transformer converted into one half-period direct current, and an alternating current of the third phase of the high voltage transformer converted into one half-period direct current connected with a cathode.
 15. A method as defined in claim 1; and further comprising using as the cathode a cup-shaped cathode with the anode located inside the cup-shaped cathode with a space between the cathode and the anode 20-40 mm.
 16. A method as defined in claim 1; and further comprising using a cathodic mode for removing from a surface layer of the metal part admixtures, fusible compounds and oxides; then using an anodic-cathodic mode; and then using an anodic pulse mode. 